The invention relates to a working machine and a method for operating a working machine.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders and articulated haulers. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other heavy working machines, such as dump trucks, excavators or other construction equipment.
A working machine is provided with a bucket, container or other type of implement for lifting, carrying and/or transporting a load. A working machine may be operated with large and heavy loads in areas where there are no roads, for example for transports in connection with road or tunnel building, sand pits, mines and similar environments.
A wheel loader is usually provided with a prime mover, such as an internal combustion engine, a transmission line, and a gearbox for supplying torque to the driving wheels of the wheel loader. In addition to supply torque to the driving wheels, the internal combustion engine has to supply energy to one or more hydraulic pumps of a hydraulic system of the wheel loader. Such a hydraulic system is used for lifting operations and/or steering the wheel loader.
Hydraulic working cylinders are arranged for lifting and lowering a lifting arm unit, on which a bucket or other type of attachment or working tool, for example forks, is mounted. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. Further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front body part and a rear body part of the wheel loader.
In order to enable a more effective operation of a working machine and lower fuel consumption, a working machine comprising an electric machine arranged between the internal combustion engine and the gearbox has been suggested in WO 2007/043924. The hybrid working machine disclosed in WO 2007/043924 has a lot of advantages as compared to conventional non-hybrid working machines using merely an internal combustion engine for propulsion and for driving the power take off unit, i.e. the hydraulic system. However, during certain operation modes the electric machine cannot be utilized in an optimized way, and the performance of the working machine may also be restricted by other components of the transmission line.
It is desirable to provide a working machine defined by way of introduction, which working machine enables the electric machine of the transmission line to be used in a more effective way and the performance of the working machine to be improved.
By the provision of a working machine provided with a gearbox which is a continuously variable transmission having a variator unit, and wherein the gearbox has an operation mode where the rotation speed of the output shaft of the gearbox is zero or close to zero independently of the rotation speed of the input shaft of the gearbox at the same time as torque can be transmitted from the input shaft to the output shaft, the electric machine can be driven within a favourable rotation speed interval. Since the gear ratio of the gearbox can be changed continuously the electric machine can be driven at the desired rotation speed substantially independently of the requisite rotation speed of the output shaft of the gearbox. Accordingly, the electric machine can be driven at an optimal rotation of speed both when used as an electric motor and as an electric generator for example during braking.
The electric machine is arranged in parallel with the prime mover, which can be an internal combustion engine, with respect to the transmission line and is mechanically connected to the transmission line between the prime mover and the gearbox, which implies a parallel electric hybridization. (Thus, the expression “in parallel with” is here a functional description, and not any description of the relative geometrical directions of the prime mover and the electric machine.) Thereby the electric machine can also be used to provide torque to a power take off unit (PTO) of the working machine. This is of great importance since the hydraulic system of the working machine is powered via the PTO, and a considerable amount of the total energy required is used for driving the hydraulic system of a working machine.
The hydraulic system can be driven by the prime mover and/or the electric machine with the rotation speed required by the hydraulic system at the same time as the gearbox can provide the requisite torque and speed of rotation for the driving wheels.
Such a continuously variable transmission (CVT) described above is usually referred to as an infinitely variable transmission (IVT), and the current mode where the rotation speed of the output shaft of the gearbox is zero or close to zero independently of the rotation speed of the input shaft of the gear box at the same time as torque can be transmitted from the input shaft to the output shaft is usually referred to as a geared neutral. The expression “zero or close to zero” is intended to cover a speed of rotation of the output shaft which is zero or in the size of a few rotations per minute. By the geared neutral function the operation of the hydraulic system can be separated from the propulsion of the working machine independently of the prime mover or the electric machine being the power source. The propulsion force can be controlled independently of the speed of rotation of the prime mover and/or the electric machine. Furthermore, the hydraulic system can be driven by the prime mover and/or by the electric machine when the working machine stands still without using any clutch for disengagement of the prime mover and/or the electric machine relative to the transmission line. In addition, the torque converter traditionally used in working machines can be omitted, since the speed of rotation can be continuously varied down to zero by means of the gearbox. A further advantage is that the electric machine can be used as a starting motor for the prime mover.
Furthermore, an electric energy storage means can be charged by means of the electric machine driven by the internal combustion engine when the working machine stands still, and potential energy of an implement such as a bucket or container can be converted into torque on the transmission line or into energy which is stored in an electric energy storage means (also when the working machine stands still).
According to one embodiment of the invention the gearbox comprises a planetary gear wheel unit enabling the power transmitted by the gearbox to be split between the variator unit and the planetary gear wheel unit. By such a gearbox having a power split (PS), in many operation modes only a minor part of the power has to be transmitted by the variator unit. The remaining power is transmitted by the planetary gear wheel unit. This implies an improved overall efficiency since the loss of energy is considerably smaller with respect to the planetary gear wheel unit compared to the variator unit. In addition, the size of the variator unit can be reduced. Preferably, a hydraulic variator is used. Generally, a hydraulic variator has improved efficiency, reduced size and is cost-effective in comparison to an electric variator or a mechanical variator.
According to a further embodiment where the working machine comprises a further electric machine, and said at least one hydraulic machine is mechanically connected to the further electric machine to enable the hydraulic machine to be driven as a pump by said further electric machine and/or to enable said further electric machine to be driven as a generator by the hydraulic machine, the hydraulic machine does not need to have a variable displacement, since the speed of rotation can be varied with the further electric machine and be adapted to the power needed in the hydraulic system. Any losses related to rotation of the hydraulic machine at an unfavourable speed of rotation can be eliminated since the speed of rotation of the hydraulic machine is not dependent on the speed of rotation of the prime mover and/or the electric machine connected to the transmission line.
The invention relates also, according to an aspect thereof, to a method for operating a working machine comprising the step of controlling the gear ratio of the gearbox to enable the electric machine to work at a substantially constant rotation speed throughout a predetermined continuous speed interval of the output shaft of the gearbox for a given value of the power level of the electric machine. Hereby the electric machine can be driven at an optimal rotation of speed, both when used as an electric motor and as an electric generator for example during braking, which in turn improves the performance of the electric machine and thereby the performance of the working machine.
According to a further embodiment of the method, energy from the hydraulic system is recovered by braking the hydraulic machine with the electric machine, and the braking torque of the electric machine is preferably controlled so as to divide the recuperated energy into a first part which is stored in an electric energy storage means connected to the electric machine and into a second part which is transmitted to the transmission line.
Thereby it is possible to optimize the recovering procedure depending on the state of charge of the electric energy storage means and the need of power for the transmission.
According to a further embodiment of the method, the gearbox is controlled to an operation mode where the rotation speed of the output shaft of the gearbox is zero or close to zero independently of the rotation speed of the input shaft of the gearbox at the same time as torque can be transmitted from the input shaft to the output shaft. Thereby the hydraulic system can be separated from the propulsion of the working machine independently of the prime mover or the electric machine being the power source. The propulsion force can be controlled independently of the speed of rotation of the prime mover and/or the electric machine.
Further advantages and advantageous features of aspects of the invention are disclosed in the following description.